(1) Field of the Invention
The present invention relates to the field of solar collectors and more particularly to solar air heaters having improved thermal efficiencies.
(2) Description of the Prior Art
A great deal of effort has been devoted by researchers in recent years to develop solar collectors for the conversion of solar energy into heat energy. These devices are potentially useful in many applications where fossil fuels are now employed as the principal sources of energy. Such applications include for example the heating of residential and commercial buildings, and the generation of electric power. Solar collectors may be widely used sometime in the not too distant future on roof tops of residential homes for supplying heat during periods of cold weather. It is of course of the utmost importance in coming years to be able to manufacture solar collectors which are relatively inexpensive and which have a high degree of thermal efficiency.
Solar collectors heretofore developed employ a collector plate for converting solar energy into heat. Typically, the collector plate is disposed inside a housing having a light-transmitting wall for passing incident solar radiation. The solar radiation passing through the wall is absorbed by the collector plate or absorber and converted into heat. The converted heat energy is then transferred to a fluid, and heats the fluid. The heated fluid is then conveyed away for storage and subsequent utilization.
In one type of solar collector the fluid to be heated is circulated through tubes or ducts for example, positioned inside or adjacent the collector plate. The collector plate in these solar collectors is usually a solid flat radiation absorbent plate, e.g., a darkened or black metal plate which absorbs the incident solar energy and transfers it as heat by conduction to the tubes or ducts where heat exchange with the fluid occurs. Solar collector devices of this type are, therefore, commonly referred to as "flat plate collectors" and they may be used to heat a liquid or gaseous medium.
When the converted heat is to be transferred only to a gaseous medium such as air, other collector designs may be used. An excellent background study of prior art solar air heaters is given in an article by A. Whillier entitled "Black-Painted Solar Air Heaters of Conventional Design", appearing in Solar Energy, Vol. 8, No. 1, pages 27-31, Pergamon Press (1964).
In one type of solar air heater the gas is passed through the housing through the collector plate, where the collector plate is a porous, gas-permeable plate, e.g. a porous black fiber mat, and the gas to be heated passes directly through the solar energy absorbing surface. Also, in this instance, the housing has an inlet and an outlet for establishing a flow path for the gas to be heated. In porous plate designs the entire collector plate acts as a heat exchange medium for transferring the absorbed or converted heat to the gas or air flowing through the device. Thus, gas or air is drawn through the inlet and flows or transpires through the collector plate and is heated. The heated gas or air exits through the outlet and is conveyed to a storage device for subsequent use. Solar collectors of this type are referred to as so-called "transpiration solar air heaters".
A major problem with solar collectors is the loss of absorbed heat by natural convection and reradiation, i.e. long-wave or infrared radiation, from the collector plate toward the light-transmitting wall.
It has been proposed in the literature to employ certain types of cellular structures such as honeycombs as a heat trap to reduce the loss of absorbed heat by natural convection in flat plate solar heaters. The heat trap is placed over the solid collector plate to guard against the development of convective heat flow away from the collector plate and toward the light-transmitting wall. Any heat that is conveyed by this convective flow to the wall can be readily lost through conduction or radiation to the outside atmosphere. Thus, Hollands in an article entitled "Honeycomb Devices in Flat Plate Solar Collectors", Solar energy, Vol. 9, No. 3, pp. 159-169, Pergamon Press (1965) discloses the use of various types of honeycomb structures, e.g., rectangular, square, triangular, etc. as a heat trap to prevent convective losses in a flat plate solar heater. The honeycomb trap can be made of glass or plastics which transmit solar rays but are opaque to long-wave radiation. In transpiration solar air heaters, of course, the loss of heat by natural convection does not occur if the gas or air to be heated continuously flows through the porous collector plate or absorber in a direction away from the light-transmitting wall.
Various attempts have also been made in the prior art to overcome the problem of reradiation losses. In flat plate collectors, spectrally selective coatings may be applied to the absorber surface to reduce reradiation losses. A review of spectrally selective coating technology is given by J. Jurisson, R. E. Peterson, and H. Y. B. Mar in an article entitled "Principles and Applications of Selective Solar Coatings" appearing in the Journal of Vacuum Science Technology, Vol. 12, No. 5, pages 1010-1015 (1975). The coatings described, however, are not effective in reducing reradiation losses from transpiration air heaters because the pores at the surface of a porous plate act as black body cavities and limit the effectiveness of any coating applied to the surface.
Various attempts have also been made in the prior art to overcome the problem of reradiation losses from transpiration solar air heaters. Thus, U.S. Pat. No. 3,102,532 to Shoemaker discloses a solar collector wherein air to be heated is passed through a gas-permeable absorber composed of multilayers of slit and expanded aluminum foil. The expanded foil is coated with a black vinyl enamel on the top surface facing the light-transmitting wall. The bright underside of the foil is highly reflective and acts as a trap to prevent loss of absorbed heat by reradiation. However, some radiation losses can occur through the openings or slits in the foil.
It has also been proposed in the literature to utilize a specularly reflecting honeycomb heat trap in a solar air heater employing a porous collector plate. Thus, Buchberg et. al., in an article entitled "Performance Characteristics of Rectangular Honeycomb Solar Thermal Convertors", Solar Energy, No. 13, pages 193-221, Pergamon Press (1971), disclose a solar air heater employing a rectangular honeycomb heat trap which is made from a specularly reflective material, e.g., aluminized paper coated with a clear resin layer.
U.S. Pat. No. 4,015,582 to Liu et al discloses a solar heat collector which employs a layer of light-transmitting insulating material which serves as an air buffer and infrared radiation trap. The combined buffer and radiation trap may be positioned between the outer surface of the collector and the radiation absorbing collector plate. The light-transmitting insulating material may be transparent glass fibers or honeycomb, for example, and is held or sandwiched between the outer collector surface and an inner non-porous layer of an air-impermeable, light-transmitting material. This solar collector design suffers from the disadvantage that the inner non-porous air impermeable layer, e.g., clear plastic or glass, is reflective of incident solar radiation and actually accounts for a significant loss in absorbed heat when the collector is operated under conditions typical of those that are encountered in space heating using solar air heaters.
U.S. Pat. No. 4,018,211 to Barr discloses a solar collector which employs at least one insulating light-transmitting honeycomb layer sandwiched between two layers of transparent material, e.g. glass, and at least one other light-transmitting honeycomb layer which serves as a heat transfer medium. The light-transmitting honeycomb layers are of such large dimension that they preclude the effective use of the honeycomb as an infra-red radiation trap.
It is an object of the present invention to provide a solar air heater employing a porous collector plate and a light-transmitting tradiation trap which has improved thermal efficiencies.
Another object of the present invention is to provide a solar air heater of the type described which is easy to assemble and economical to manufacture.